Methods such as these are used, for example, in an apparatus for reading from and/or writing to optical recording media having wobbled tracks, in order to obtain address information from the wobbled tracks (ADIP information, address in pregroove) or to use the wobble frequency to produce a write clock.
In general, in optical recording media, which are in the form of discs and are suitable for reading from and/or writing to, the tracks are formed such that they represent an interleaved spiral or concentric circles. Especially in the case of optical recording media which are suitable for writing to, the tracks additionally are wobbled in a specific form, in order to find specific positions on the medium. This means that the track is not an approximately straight line, but a serpentine line. By way of example, the shape of this serpentine line can contain address information, which is used for identifying a specific position on this optical recording medium. Various methods are used for coding, examples of which include frequency modulation or phase modulation. Furthermore, the wobble signal may also be used for rotation speed information or for presetting a write data rate.
For high density optical recording media, it has been proposed to modulate the wobble signal using two methods in an intermixed manner: Minimum Shift Keying cosine variant (MSK-cos) and Harmonic Modulated Wave (HMW). Only some of the wobble periods are modulated. Most of the wobble periods are monotone wobbles (MW). The MSK-cos method is mainly adopted for the ADIP unit synchronization, replacing three wobble periods by one MSK mark. This is illustrated in FIG. 1. The MSK mark indicates the start of the ADIP unit or is used for synchronisation or data recognition. The HMW method is mainly employed for the ADIP data. The second harmonic of the fundamental wobble frequency is added to the wobble with a lower amplitude level. Its phase is in quadrature with the fundamental wobble frequency and it is bi-phase modulated according to the ADIP bit, which is illustrated in FIG. 2.
The two methods are not used separately for synchronization and ADIP information, as illustrated in the FIG. 3, which shows the different units ocurring in an ADIP word. On the one hand, in the ADIP word another MSK mark is added at different locations, for the data_0 unit at the wobble periods 14,15, and 16, and for the data_1 unit at the wobble periods 12,13, and 14. Therefore, MSK marks could be used also for ADIP data demodulation. On the other hand, some ADIP units, the so-called reference units, also have the second harmonic wobble frequency. In this case, the second harmonic has a fixed phase shift, so the reference unit could be used for the synchronization of the ADIP nibble.
Since the above described modulation of the wobble signal is quite new, solutions for a reliable wobble demodulation are hardly known. Typical schemes known from prior art for frequency or phase demodulation could be used, but it is difficult to apply the proper combination of both schemes. If only pure frequency demodulation or pure phase demodulation are used, a significant part of the signal energy is lost. This results in an undesirable performance degradation.
Kobayashi et al. in Jpn. J. Appl. Phys Vol. 42 (2003), pp 915–918, propose a method for detecting the MSK marks and the HMW sawtooth wobble. A heterodyne circuit consisting of a carrier multiplier, an integrator and a sample-and hold element is used for this purpose. The wobble signal is multiplied by the cosine carrier of the fundamental frequency for detecting the MSK marks in the multiplier. On the other hand it is multiplied by the sine carrier of the second harmonic frequency for detecting the HMW sawtooth wobble. However, the proposed method does only use a fraction of the available signal energy.
It is, therefore, an object of the invention to propose a method for a reliable wobble demodulation, which overcomes the above mentioned problems.